The present invention relates to a recording medium, and particularly to an ink jet recording medium suitable for using in the case of carrying out bidirectional printing using a printer of a serial system.
In recent printers, increasing the high resolution proceeds and there is a tendency to increase the directing amount of ink per unit area. Particularly, in the image region of an intermediate tone, because a large amount of light-color ink such as a photo ink, etc., is used for reducing a particular feeling, the directing amount of ink per unit area is increased. Thus, as a recording medium used for these printers, a recording medium having a good absorption (large absorbing capacity) has been desired.
For realizing high-resolution printing, it is necessary to increase the density of nozzles and increase the frequency for jetting ink. Also, for realizing high-speed printing, it is necessary to carry out so-called bidirectional printing of jetting ink droplets at the forward path and backward path and increase the above-decried frequency for jetting ink. On the other hand, the printing speed depends upon the scanning speed of a head. (In addition, in the invention, xe2x80x9cthe scanning speed of headxe2x80x9d means xe2x80x9cthe time until the head returns to the same positionxe2x80x9d).
Accordingly, in a high resolution mode, by all means, the scanning speed becomes slow, and thus for increasing the printing speed, the necessity of printing from bidirections occurs.
In the bidirectional printing, the jetting order of inks becomes reverse between forwarding and backwarding, and thus, in bidirectional printing using parallel heads suitable for high speed printing, when the ink-absorbing capacity of the surface layer of an ink-receiving layer is small or the ink-absorbing speed is slow, in the case of printing mixed color images of, for example, red, blue, and green colors, coloring becomes different between the forward path and backward path. (In addition, in the invention, xe2x80x9cthe ink-absorbing speedxe2x80x9d mean xe2x80x9cthe time until attaching an ink droplet to a surface to vanish the ink droplet from the surfacexe2x80x9d).
For solving the problem in bidirectional printing, it is effective to improve the absorption of ink. For improving the absorption of ink, largely, the following two methods are considered.
One of them is a method of forming an ink-receiving layer containing a pigment having a large specific area, such as synthetic silica, etc., as the main constituent at a thickness of at least a definite layer thickness as disclosed in JP-A-52-9074, JP-A-58-72495, JP-A-55-51583, etc. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d.)
According to the above-described method of prior art, a high ink-absorption can be insured, but because inks permeate deeply, there are problems that the printing density is low and only images lacking in the clearness are obtained. Particularly, since the permeability of inks into a recording medium is increased for intending the reduction of the occurrence of bleeding between colors, in the recent full color printer of lowering the surface tension of inks, lowering of the printed density is remarkable.
In another method, the wetting property of the surface is improved as disclosed in JP-A-63-39373, etc.
In the method, when the directing amount of ink is small because the resolution in low, etc., the absorption is insured to some extent but when the resolution is high or a dense image is printed, and also when the directing amount of ink is large, there are problems that because essentially the absorbing amount of inks is insufficient, the inks overflow to bleed images and also because dots are large, only images lacking in sharpness are obtained.
On the other hand, when the ink absorbing speed is fast and the contact angle of inks to the surface of a recording medium is high, dots become small and inks permeate to height (depth) direction, whereby there are problems that coloring property is poor and only images inferior in the color reproducibility are obtained.
On the other hand, when the ink-absorbing speed is slow and the contact angle is low, dots are large, whereby there are problems that resolution becomes low, fine portions are broken, and letters, etc., become thick and bleed.
Also, when the ink-absorbing capacity of an ink-receiving layer is small or an ink-absorbing speed is slow, there is a problem that for example, in the case of expressing a green color with a yellow ink and a cyan ink, the case of printing cyan after printing yellow (forward path) and the case of printing yellow after printing cyan (backward path) give a different tone of green. This is caused that when in the state of remaining a first ink on the surface, the next ink is overlapped, the latter ink cannot stay on the surface layer and permeates into the ink-receiving layer deeper than the first ink.
In this case, when an image of at least twice the width (the printing width which can be printed by one scan) of a head, color unevenness (lateral stripes) occurs on the image, which becomes a problem.
As described above, until now, a recording medium, which does not cause color unevenness by printing at a high resolution mode and bidirectional printing for high-speed printing in the case of printing by a full color printer, and also is excellent in the high image quality (high resolution) and in coloring, cannot be provided.
The present invention has been made in view of the above-described problems, and an object of the invention is to provide an ink jet recording medium which is excellent in the ink absorption, gives a high coloring property of images, does not cause color unevenness and can print images of a high image quality even by bidirectional printing by a high-resolution printer.
As the result of investigations about the relation of the image quality of recorded images and the ink-absorbing capacity of an ink-receiving layer in a recording medium, the wetting property of inks to an ink-receiving layer, the absorption speed of inks into an ink-receiving layer, the scanning speed of head, etc., the present inventors have discovered that when a recording medium, wherein the ink-receiving layer has a specific ink-absorbing capacity and the contact angle of ink to the surface of the ink-receiving layer has a specific value, is used, images having a high image quality are obtained and have accomplished the present invention.
That is, the gist (first embodiment) of the ink jet recording medium of the present invention is an ink jet recording medium in the case of carrying out bidirectional printing using a printer of a serial system wherein inks of at least two colors are mounted in carriages and the nozzle row is disposed in parallel with the main scanning direction of a head, wherein the recording medium comprises a base material and an ink-receiving layer, the ink-receiving layer has an average ink-absorbing capacity of from 10 to 30 cc/m2, and the contact angle of the uppermost surface layer of the ink-receiving layer is from 20 to 60 degree to an ink having a surface tension of from 25 to 72 dyn/cm, whereby the ink jet recording medium as the object of the invention can be provided.
Also, in the ink jet recording medium of the invention as the second embodiment, the ink-receiving layer is composed of at least two layers (that is, comprising at least two layers constituted of an upper layer forming the uppermost surface layer and at least one lower layer) and the ink absorbing capacity of the uppermost surface layer is larger than the ink-absorbing capacity of the lower layer.
Also, in the ink jet recording medium of the invention as the third embodiment, the ink absorbing speed of the uppermost surface layer of the ink-receiving layer is faster than the scanning speed of the head.
Furthermore, in the ink jet recording medium of the invention as the fourth embodiment, when the thickness of the uppermost surface layer is xe2x80x9cA (xcexcm)xe2x80x9d, the average ink absorbing capacity of the uppermost surface layer is xe2x80x9cLa (cc/m2)xe2x80x9d, the thickness of the whole layers of the ink-receiving layer is xe2x80x9cT (xcexcm)xe2x80x9d, and the average ink absorbing capacity of the whole layers of the ink-receiving layer is xe2x80x9cL (cc/m2), the following relations of equation (1) and equation (2) are satisfied:
5 xcexcm less than A less than 0.7Txe2x80x83xe2x80x83Equation (1) 
1.1xc3x97L less than La less than 2xc3x97Lxe2x80x83xe2x80x83Equation (2), 
whereby
an ink jet recording medium which can print images of a high image quality without causing color unevenness even by, particularly, bidirectional printing using a high-resolution printer can be provided.
Moreover, in the ink jet recording medium of the invention as the fifth embodiment, the ink-receiving layer contains a pigment, and the pigment has an apparent specific volume of from 3 to 20 cm3/g and is at least one selected from synthetic silica, aluminum hydroxide, alumina, boehmite, calcium carbonate, aluminum silicate, magnesium carbonate, zeolite, and zinc oxide.
In the ink jet recording medium of the invention as the sixth embodiment, the ink-receiving layer comprises a coated layer containing a pigment for absorbing an ink and a binder in an amount of from 20 to 80 parts to 100 parts of the pigment, and the coated amount of the coated layer is from 10 to 30 g/m2, whereby the ink-receiving layer having an average ink-absorbing capacity of from 10 to 30 cc/m2 can be provided.
Also, in the ink jet recording medium of the invention as the seventh embodiment, the uppermost surface layer contains a cationic sizing agent in an amount of from 0.2 to 8 parts to 100 parts of the pigment for absorbing an ink.
In the ink jet recording medium of the invention as the eighth embodiment, the above-described uppermost surface layer is formed by overcoating a layer containing from 0.2 to 3 parts of a cationic sizing agent.
Also, in the ink jet recording medium of the invention as the ninth embodiment, the cationic sizing agent is made of a styrene-base resin, whereby the contact angle of the uppermost surface layer of the ink-receiving layer can be set to from 20 to 60 degree to an ink having a surface tension of from 25 to 72 dyn/cm, and also the ink jet recording medium, wherein the ink-absorbing capacity of the uppermost surface layer is larger than the ink-absorbing capacity of the lower layer, can be provided.
Also, the ink jet recording medium of the invention is used for a recording method wherein the resolution is at least 720xc3x97720 dpi as the tenth embodiment and is also used for a recording method that the maximum jetting amount of ink per one dot is not larger than 15 picoliters (pl) as the eleventh embodiment.
Even in the recording methods of such a resolution and the maximum jetting amount of ink, images of a high image quality can be printed without causing color unevenness.